1,4-diketo-3,6-di(4-biphenylyl)-pyrrolo [3,4 c]-pyrrole pigment is known for its high chroma and outstanding weatherfastness properties. Its use in automotive paint applications is well-known. However, the use of 1,4-diketo-3,6-di(4-biphenylyl)-pyrrolo [3,4 c]-pyrrole as a pigment in high performance plastics, for example polar engineering plastics, is restricted because it does not possess sufficient heat stability for use in such substrates.
The use of 2,9-dichloroquinacridone having a surface area below 30 m.sup.2 /g as a pigment for engineering plastics is known. The use of pigment compositions containing 2,9-dichloroquinacridone compositions and bis-(4,5,6,7-tetrachloro-isoindolin-1-on-3-ylidene)-phenylene-1,4-diamine for pigmenting engineering plastics is also known. Although these pigments show outstanding heat stability, they are not suitable for all applications because they have weak color strength and cover only a magenta to bluish-red color space.
U.S. Pat. No. 4,810,304 discloses solid solution pigments containing quinacridone and diketopyrrolopyrrole components. When compared to the corresponding physical pigment mixtures, such pigment solid solutions show unique and generally improved pigment properties.
This invention relates to the surprising discovery that physical mixtures containing proportions of 1,4-diketo-3,6-di(4-biphenylyl)-pyrrolo [3,4 c]-pyrrole and 2,9-dichloroquinacridone within certain ranges show advanced pigment properties without pigment solid solution formation. The new pigment compositions possess excellent dispersibility, high color strength, high weatherability, high saturation and outstanding heat stability in engineering plastics.